Underreamers are employed in well drilling operations to enlarge a pilot hole. In casing drilling, the drill string is made up of the casing that will be eventually cemented in the well. If the drill bit is retrievable, it will be part of a bottom hole assembly that latches to a collar or profile sub located near the bottom of the string of casing. The bottom hole assembly extends below the string of casing, and the drill bit is on its lower end for drilling a pilot hole. The underreamer is located above the drill bit for enlarging the pilot hole to an outer diameter greater than the outer diameter of the string of casing.
The underreamer has arms that are pivotally mounted to the body of the underreamer for moving between retracted and extended positions. Cutters, typically polycrystalline diamond disks, are mounted to the leading face of each arm. One type of underreamer has an actuator mandrel carried in its longitudinal passage, the actuator mandrel being axially movable from an upstream position to a downstream position in response to drilling fluid being pumped down the drill string. The actuator mandrel is cooperatively engaged with the arms for moving the arms to an extended position when the actuator mandrel moves to the downstream position.
The string of casing is rotated by a casing gripper and a top drive of the drilling rig. The bottom hole assembly may include a drill motor that rotates the underreamer and the drill bit independently of the casing string. During drilling, drilling fluid is pumped down the casing string, through the bottom hole assembly and out nozzles of the drill bit. The drilling fluid flows back up the borehole past the underreamer and up the annulus surrounding the string of casing. The drilling fluid removes cuttings and provides lubrication and cooling of the drill bit and underreamer. Nevertheless, in some formations, the cutters on the underreamers arms can become clogged with cuttings and operate at elevated temperatures. Elevated temperatures may be detrimental to the performance and the resistance to abrasion.